Using dogs as a model animal the pharmacokinetics (kinetics of distribution and elimination) of methotrexate (MTX), a potent widely used anticancer drug, will be thoroughly studied by a new, simple, rapid, specific and micro high-performance liquid chromatographic (HPLC) method for simultaneous determinations of MTX and its two active metabolites, 7-hydroxy-methotrexate (7-OH-MTX), and 2,4-diamino-N10-methylpteroic acid (DMA), in biological fluids. The proposed pharmacokinetic studies will include: the distribution kinetics of MTX and metabolites between plasma and blood cells, and possible metabolism in blood cells and their implications in proper methods of collection of plasma or serum samples from patients for monitoring of MTX and metabolites; differences in arterial plasma and venous plasa levels of MTX and its metabolites after dosing and their clinical implications; changes in elimination (metabolism and excretion) kinetics following repeated intravenous dosing; changes in pharmacokinetics over a wide-dose range (1-1000mg/kg); pharmacokinetics of 7-OH-MTX and DMA; mechanisms of renal excretion of MTX and metabolites by studying their renal clearances at different doses and steady-state plasma MTX levels; effects of hydration and urinary alkalinization on renal clearance of MTX and metabolites. An insignificant effect may suggest that these current practices could be discontinued much earlier in patients; effects of various degrees of induced renal impairment on pharmacokinetics (with emphasis on accumulation) of MTX and metabolites. The results may suggest a basis of dosage regimens of MTX in patients with renal failure. At preset these patients are usually not treated with MTX probably due to a lack of pharmacokinetic information; use of oral activated charcoal and cholestyramine as potential detoxifying agents in MTX therapy. Their advantages (as compared to other existing methods) are safe, simple, convenient, and economical. They may be employed to reduce MTX levels in gastrointestinal tract, and thereby decrease GI side effects during therapy. The micro HPLC method developed should be useful for clinical pharmacokinetic studies of MTX in patients especially in pediatrics. The pharmacokinetic results obtained from the present study in dogs should be valuable to future pharmacokinetic and pharmacodynamic (Text Truncated - Exceeds Capacity)